1. Field of the Invention
The present invention relates to an inferior-and-superior-limb blood-pressure-index measuring apparatus which measures an inferior-and-superior-limb blood-pressure index of a living subject.
2. Related Art Statement
An inferior-and-superior-limb blood-pressure index is generally known as a ratio of a blood pressure of a superior limb of a living subject to a blood pressure of an inferior limb of the subject, or a ratio of the blood pressure of the inferior limb to that of the superior limb. Respective systolic blood pressures of the inferior and superior limbs are generally used as the respective blood pressures of those limbs. In addition, generally, an ankle is selected as the inferior limb and an upper arm is selected as the superior limb, so that an ankle-and-upper-arm blood-pressure index is determined as the inferior-and-superior-limb blood-pressure index. The inferior-and-superior-limb blood-pressure index represented by the ankle-and-upper-arm blood-pressure index is useful in making a diagnosis about stenosis or obliteration caused by atheromatous arteriosclerosis (atherosclerosis). If a living subject has stenosis in a portion of an artery, subject's blood pressure lowers on a downstream side of the stenotic portion, which leads to showing an abnormal inferior-and-superior-limb blood-pressure index. Thus, stenonis of artery can be diagnosed based on inferior-and-superior-limb blood-pressure index. In many cases, stenosis of artery occurs to inferior limbs of a human person.
Since inferior-and-superior-limb blood-pressure index is a simple ratio, i.e., a ratio of one of inferior-limb blood pressure and superior-limb blood pressure to the other, reliable measurement of inferior-and-superior-limb blood-pressure index requires that inferior-limb blood pressure and superior-limb blood pressure be measured with accuracy. There is known a different sort of arteriosclerosis than atherosclerosis, that is, “calcification”. If calcification of an artery advances, blood pressure in the artery increases because of hardening of the wall of artery. If the calcification of the artery further advances, the artery cannot be completely closed and the blood pressure further increases. Thus, even if an artery of an inferior limb has stenosis, a normal inferior-and-superior-limb blood-pressure index value may be obtained if the artery suffers advanced calcification.
Hence, there has been proposed an inferior-and-superior-limb blood-pressure-index measuring apparatus which simultaneously displays an inferior-and-superior-limb blood-pressure index, and pulse-wave-propagation-velocity-related information, such as a pulse-wave propagation velocity, or a corrected pulse-wave propagation velocity which is obtained by correcting a pulse-wave propagation velocity so as to correspond to a pre-selected blood pressure, so that when the inferior-and-superior-limb blood-pressure index being displayed is normal, it can be judged by a medical person whether the normal index means that the inferior limb's artery does not have stenosis or that the artery has not only stenosis but also advanced calcification. This apparatus is disclosed in Japanese Patent No. 3,140,007 or its corresponding U.S. Pat. No. 6,355,000. Pulse-wave-propagation-velocity-related information, such as a pulse-wave propagation velocity, can be used to evaluate a degree of calcification of artery. Since the apparatus simultaneously displays an inferior-and-superior-limb blood-pressure index and pulse-wave-propagation-velocity-related information, an accurate judgment can be made about whether a patient has arteriostenosis. In the following description of the present application, pulse-wave propagation velocity and pulse-wave propagation time, and corrected pulse-wave propagation velocity and corrected pulse-wave propagation time which are obtained by correcting pulse-wave propagation velocity and pulse-wave propagation time, respectively, so as to correspond to a pre-selected blood pressure, are all defined as pulse-wave-propagation-velocity-related information.
A living subject who has normal blood vessels, such as a young person, has substantially no difference between his or her inferior-limb and superior-limb blood pressure values, and accordingly an inferior-and-superior-limb blood-pressure index value of the subject should be around one. However, in some cases, a measurement error or malfunction may abnormally lower the inferior-limb blood pressure, thereby showing an abnormal inferior-and-superior-limb blood-pressure index. However, the above-indicated conventional inferior-and-superior-limb blood-pressure-index measuring apparatus cannot judge, when the blood-pressure index is abnormal, whether the abnormal index has resulted from the stenosis of inferior limb or the abnormality of blood-pressure measurement. Pulse-wave-propagation-velocity-related information changes in relation with calcification of artery and, at the same time, changes in relation with stenosis of artery. However, calcification of artery and stenosis of artery influence pulse-wave-propagation-velocity-related information in “opposite” directions. More specifically described using pulse-wave propagation velocity as a sort of pulse-wave-propagation-velocity-related information, as the calcification of artery advances, the propagation velocity increases but, as the stenosis of artery advances, the propagation velocity decreases. Therefore, if an artery simultaneously suffers advanced calcification and stenosis, then the pulse-wave-propagation-velocity-related information obtained from the artery may be normal, or around a normal range. It is natural that pulse-wave-propagation-velocity-related information obtained from a living subject whose blood vessels are normal should be normal. Thus, the above-indicated conventional inferior- and superior-limb blood-pressure-index measuring apparatus cannot judge, when a measured inferior-and-superior-limb blood-pressure index is abnormal, whether the abnormal index has resulted from stenosis of inferior limb or abnormality of blood-pressure measurement.
In addition, normal range of pulse-wave-propagation-velocity-related information largely changes among individual subjects, and more or less changes on each subject depending on his or her physical condition. However, the inferior-and-superior-limb blood-pressure-index measuring apparatus disclosed by the above-indicated Japanese Patent No. 3,140,007 obtains only a single sort of pulse-wave-propagation-velocity-related information so as to recognize lowering of reliability of inferior-and-superior-limb blood-pressure index that is caused by calcification of artery. In this case, a medical person must evaluate the blood-pressure index while taking into account the individual differences of patients and/or the physical condition of each patient, and accordingly cannot make a sufficiently accurate evaluation.